The present invention relates generally to rapid battery chargers, and more particularly to a rapid battery charger having a charge rate of more than 5 C where C is a battery charging rate and represents a charging current required for charging a battery in an hour.
Recently, battery-driven cordless products which do not require a power supply cord have been extensively used in various fields. In the field of electrically-driven machine tools, cordless machine tools have become extensively used. Such cordless machine tools typically use a nickel-cadmium battery as a power source battery whose internal resistance is so small that a large amount of current can be supplied therefrom and whose service-life is relatively long under the repetitive charges and discharges.
To perform a rapid charge of the battery, a battery charger having a charge rate of 1 C has been of a major use. Recently, the battery charger having a charge rate of 5 C has recently been on sale.
However, the service life of the nickel-cadmium battery tends to be extremely shortened when the battery is quickly charged under a low temperature circumstance, say, less than 0.degree. C. Because an electrolyte is liable to leak out of the battery and an insulating material used in the battery is liable to be damaged due to an abrupt increase of an inner gas pressure of the battery. For such reasons, consumers are warned not to perform quick charge of the nickel-cadmium batteries in the low temperature circumstance. This means incapability of using the battery-driven devices under such a circumstance.